1. Field of the Invention
The present invention relates to a liquid-crystal display device and, more particularly, to a voltage conversion circuit that converts an output voltage from a data driver used when a liquid-crystal display device is driven so as to invert dots.
2. Description of the Related Art
Generally speaking, dot inversion driving, which is one of the methods for driving a liquid-crystal display device, is a driving method which features high display quality, such as high contrast or low crosstalk. In this driving method, however, when a standard liquid crystal is used, a driving voltage of .+-.5 V is used to invert each dot, and therefore, a data driver capable of outputting about 10 V is required. In this type of data driver IC suitable for use with a high voltage, finer patterning of internal elements is difficult, hampering improvements in speed, reduction of size, and lowering of cost of the liquid-crystal display device.
Therefore, instead of using a data driver IC suitable for use with a high voltage, capable of outputting about 10 V, by using, for example, a combination of a data driver IC of a single +5 V output and a switched capacitor circuit, it is possible to easily form a polarity inversion circuit capable of outputting a driving voltage of -5 V, which is of a polarity opposite to that of the data driver IC.
The switched capacitor circuit, as shown in, for example, FIG. 8, comprises transistors M1, M2, M3, and M4, and a capacitor Cb. On/off control of the transistors M1 and M4 is performed in accordance with a clock signal .phi.a, and on/off control of the transistors M2 and M3 is performed in accordance with a clock signal .phi.b. That is, the transistors M1 and M4 are turned on when the clock signal .phi.a is at a "high" level and turned off when the clock signal .phi.a is at a "low" level. Also, the transistors M2 and M3 are turned on when the clock signal .phi.b is at a "high" level and turned off when the clock signal .phi.b is at a "low" level. As shown in FIG. 9, these clock signals .phi.a and .phi.b have a phase difference of 180.degree. at the same cycle, and are formed such that they do not reach a "high" level simultaneously. Reference numeral C2 denotes a data-line parasitic capacitor.
Therefore, in FIG. 9, in the period in which the clock signal .phi.a is at a "high" level and the clock signal .phi.b is at a "low" level, the transistors M1 and M4 are turned on, causing the side connected to the transistor M1 of the capacitor Cb to be charged to a Vin level and the side connected to the transistor M4 of the capacitor Cb to be charged to a Vsc level. Next, when the clock signal .phi.a reaches a "low" level and the clock signal .phi.b reaches a "high" level, the transistors M2 and M3 are turned on, causing the side connected to the transistor M2 of the capacitor Cb to reach a Vsc level. As a result, Vsc-Vin is output as a Vout level from the side connected to the transistor M3. That is, when Vin=+5 V and Vsc=0 V, Vout=Vsc-Vin =-5 V is output, making it possible to invert the output voltage of the data driver IC. In actuality, there is an example in which this switched capacitor circuit is used to generate an inverse-polarity constant voltage for driving a STN liquid crystal.
However, in the above-described conventional switched capacitor circuit, there is a problem in that the output impedance is large, and it is difficult to directly drive a load by itself. Therefore, in order to solve this problem, the switched capacitor circuit is used only to generate an inverse-polarity constant voltage, and in order to prevent an increase in the output impedance of the switched capacitor circuit, a method is conceived in which a buffer capacitor is provided separately. However, if a buffer capacitor is added, a problem arises in that response speed is decreased, and it is not appropriate to use the buffer capacitor for image signals.
As another method of decreasing output impedance, generally speaking, a method is conceivable in which an impedance conversion circuit by an operational amplifier is used. However, it is not possible to form this circuit, for example, from an amorphous Si thin-film transistor (hereinafter referred to as an aSi-TFT), which is often used as a switching element of an active-matrix-type liquidcrystal display device. The reason for this is that, in an aSi-TFT, the mobility of carriers is small and a predetermined response speed cannot be obtained.
Both methods require that a buffer capacitor, an impedance conversion circuit, or the like, be mounted separately on a substrate of a liquid-crystal display device. This is an obstacle to size reduction and power consumption reduction of the liquid-crystal display device.